At the current state of the, art, input and output devices associated with a computer operate at an increased rate. Accordingly, a high speed of operation is required of a printer which is to be used as an output device. It should be understood that the printer be capable of providing a high resolution for the images being recorded, but it is also necessary that the printer be compact to reduce the space requirement and facilitates its maintenance. If a high speed of printing operation is realized, this does not assure an actual high speed operation if the apparatus is bulky and it takes a long time for trouble-shooting. Another factor required is a low running cost.
When a record in the form of an endless belt is used, the record extending around the belt rollers must be changed from time to time because of degradation or damage thereof. It will be understood that when the record unit is not mounted on the printer or during its stowage or shipment, an unnecessary application of the tension to the record causes a change in the configuration of the record by producing a curvature in its regions which are pressed against the belt rollers. In addition, if the tension is maintained under adverse environment of high temperature, high humidity and/or low temperature and over a prolonged period of time as experienced during the stowage for shipment, an elongation, surface cracks or degradation of quality of the record may result. Accordingly, a tension release mechanism is provided to avoid these problems in order to release the action of the tensioning mechanism during the replacement, stowage or shipment of the record.
As the record in the form of an endless belt moves along a loop, a static electricity may be produced between the record and a member which supports it. The static electric tends to attract the record to the support member, interfering with a smooth movement of the record.
When a photosensitive member comprises an endless belt which is supported by a drive roller and one or more follower rollers, each of which has a peripheral length that is reduced as compared with the full perimeter of the belt, the space occupied by the photosensitive member within the printer can be reduced, thus resulting in a reduced size of the arrangement. However, with this arrangement, the photosensitive member will have a reduced radius of curvature in its region of contact with the roller or rollers and a slack may be produced in the photosensitive belt between adjacent rollers in a selected region. Therefore, the locations for the exposure station, the charger and the developing unit must be carefully chosen. Specifically, the exposure station must be located in a region of the record where a fluctuation in the running speed of the photosensitive surface is low and having an increased radius of curvature. The developing unit must be located so as to minimize a variation in the developing gap. Finally, the location for the charger must be chosen in a region of the record which is free from a rapid variation in its configuration and where a variation in the position of the photosensitive surface is reduced.
To form a sharp image, it is essential that an electrostatic latent image be formed in accurate conformity to image information. At this end, it is necessary that the surface of the record which has been uniformly charged as a result of a corona discharge from the charger be rendered conductive only in those regions which are exposed and the electric potential of such regions be brought as close to the ground potential or zero volt as possible.
In the above case, it is necessary that the record be connected to a grounding brush in an electrically effective and stable manner.
As the record in the form of the endless belt is driven by the roller, a meandering and an offsetting phenomenon of the belt may occur in which the record shifts in a direction perpendicular to the direction of the drive applied, and which is attributable to a difference in the magnitude of the tension applied to the record by the roller, as measured between the opposite axial ends of the roller, or to an error in the parallelism between the shafts of the rollers. Such phenomenon must be prevented since otherwise an image formed on the record or to be transferred onto a transfer sheet becomes offset.
In one arrangement which is used to prevent this phenomenon, the roller or rollers which support and drive the record in the form of the endless belt is provided with flanges which control the edges of the record to limit the meandering and offsetting of the belt. In another arrangement, at least one of the rollers which support and drive the endless belt is provided with an automatic aligning mechanism including an aligning shaft which utilizes an offsetting force of the belt produced, as the offset occurs to cause the belt to be driven in the opposite direction from the direction of initial off-setting. In the first mentioned arrangement, stresses are produced in the edge of the record as a result of an offsetting action, and produces a deformation in the edge thereof, thus substantially degrading the durability of the record and the reliability of the printer. Hence, when this arrangement is used, the record must have a base of an increased thickness to increase the resistance to deformation. Alternatively, the offsetting action of the belt must be diminished. However, an increased thickness of the base of the record is undesirable since it causes a reduction in the bonding strength of the record layer to the base and an increase in the belt tension as a result of increasing bending stresses. If one elects the choice to diminish the offsetting action of the belt, a delicate adjustment of the tension in the belt is required, resulting in a compled arrangement which requires a high precision. In the second arrangement mentioned above, a roller assembly of a high accuracy is necessary to permit a reliable automatic aligning operation. Again, there results a complex and expensive arrangement which is of an increased size.
If an offset control is performed during the time when a transfer operation is being performed, the movement of the record and the transfer sheet in the respective given directions will be disturbed, disadvantageously producing back stripes or blurring of transferred image. Hence, the offset control should be made when no transfer operation takes place or when the transfer charger is inoperative.
To control an offsetting of the belt, the use of some means which constrains the lateral edge of the belt involves the likelihood that such edge may be deformed or damaged.
The printer according to the invention adopts a magnetic brush developing process in which a magnetic developer is used to convert an electrostatic latent image into a visual image. As is well recognized, in a magnetic brush development process, a developing roller is formed by a non-magnetic cylindrical sleeve and a magnet or magnets disposed inside the sleeve. By producing a relative movement between the sleeve and the magnets, a magnetic brush is formed on the surface of the sleeve for sliding contact with the latent image to convert it into a visual image. The magnetic brush developing process is most popular among the developing processes of dry type in view of its practical feasibility, and is extensively used in copying machines, printers, plotters, and recording systems of facsimile systems.
In the magnetic brush developing process, the toner concentration must be uniform and the height of tuft of the magnetic brush must be substantially constant in order to enable a developing effect free from non-uniformity. Where two-component developer is used, a sufficient agitation must be made to achieve a uniform toner concentration. To form a tuft of the magnetic brush which has a constant height, there must be provided a doctor blade which is mounted at a given spacing from the sleeve surface, thus limiting the developer to a given height as it is formed on the sleeve.
In the prior art practice, the spacing between the sleeve and the doctor blade is on the order of 0.1 to 1.0 mm where one component developer is used in the magnetic brush developing process, and such spacing is on the order of 1.0 to 3.0 mm where two-component developer is used. When the spacing between the sleeve and the doctor blade is small as given above, a white streaking may appear on a copy image.
A study made by the present inventors revealed that the space between the sleeve and the doctor is plugged with particles of paper, dust, metal powder or the like in the region of white streakings. When the developer recovered by the cleaning unit is returned to the developing unit for re-use, particles of paper, metal powder and the like are admixed with the developer. Also, particles of paper, dust and metal powder which are dispersed within the printer may also become admixed with the developer. As a result, if the clearance between the sleeve and the doctor blade is too small, the space may be plugged with such impurities to prevent a free passage of the developer therethrough, resulting in a localized reduction in the amount of developer. This results in a reduced height of tuft of the magnetic brush where no contact occurs with the latent image or the pressure of contact is minimized if the contact occurs at all, resulting in so-called white streaking. Obviously, a similar streaking also occurs as a result of agglomeration of developer. The occurrence of such streaking is particularly notable with one-component developing process which is utilized with a reduced spacing between the doctor blade and the sleeve.
It is a feature of one-component developing process that the volumetric occupancy of magnetic powder in the magnetic toner is very small, with consequence that the magnetization per single particle of toner is low and that when a magnetic brush is formed on the developing sleeve by means of magnets disposed therein, such brush cannot be comparable to a rough and elongate brush as may be formed with a carrier in two-component developing process. If a toner layer of an increased thickness is formed, the toner formed is susceptible to non-uniformity, giving rise to non-uniform optical density of the image. Hence, it is essential that a magnetic brush of a reduced thickness be uniformly formed on the developing sleeve when the one-component developer is used.
In addition, since the layer has a reduced thickness, once the developer layer is used in the developing step, there is produced a great difference in the thickness of the developer layer in areas corresponding to the image and the remainder. If the developer layer is allowed to remain on the sleeve and a fresh developer is supplied thereto, there is a difference in the characteristic between the developer remaining on the surface and the fresh developer, preventing a uniform layer from being formed again to thereby cause a non-uniform optical density of the image or an after-image. An increased amount of triboelectricity of the residual toner increases the electrostatic attraction to the developing sleeve, making it difficult for the toner to be separated from the sleeve.
To overcome these disadvantages, it has been proposed in the prior art to provide a scraper which removes any residual toner from the developing sleeve. Alternatively, a scraper blade may be apertured to return the toner once removed again onto the developing sleeve. However, when the scraper is used, it is usually arranged for contact with the developing sleeve, which requires a troublesome fine adjustment of the mounting of the scraper. Also, the scraper may impair the sleeve. Additionally, the urging effect of the scraper upon the toner may promote the agglomeration of the toner. A smooth removal of the toner may be prevented if the scraper undulates.
Where an apertured scraper is used, it is disposed for contact with the developing sleeve, and the toner once removed from the sleeve is returned thereto again through apertures, thus achieving a toner stirring action. However, the contact of the scraper blade with the sleeve prevents its proper functioning if the blade undulates to be separated from the sleeve. Also, the blade or sleeve may be impaired or an agglomeration of the toner may result. Where a number of copies are produced from a single original, if the toner is removed once from the developing sleeve, it may be supplied to the developing sleeve again before it is sufficiently stirred with fresh developer contained within a toner hopper, thus causing a non-uniform optical density of the image and an after-image.
The replenishment of developer takes place at shorter intervals, and hence its frequency is greatly higher than the interval with which the record is normally changed.
It will be seen that the record cannot be changed unless the unit is completely taken out of the printer, but that the replenishment of developer can be effected if the unit is withdrawn to a degree which is sufficient to open the top cover of the vessel. On the other hand, it is desirable that the exposure of the record to the outside of the arrangement be avoided or minimized since such exposure may cause a degradation in the photosensitive response by the indoor illumination, scratches or deposition of dirts or dusts thereon.
When a record in the form of an endless belt is used, a slack or undulation may occur in the record across the belt rollers. Accordingly, the disposition of the various devices around the record must be carefully chosen.
A higher precision of relative position with respect to the record, as compared with that for the developing unit, is required of the exposure unit. A best choice to maintain a constant relative position between the record and the exposure unit will be obtained by performing the exposure of the record around the roller or in the so-called curved region E. In this instance, it is prerequisite that the scan line of a scanning beam be parallel to the axis of the roller. If such parallel relationship is not maintained for a roller of a reduced diameter, the exposure will not be uniform in the axial direction of the roller or crosswise of the record.
It is also necessary that the charger be disposed at a location which is not subject to a variation in its relative position with respect to the record. In particular, with the charger of Scorotron type, the plurality of grid wires must be all disposed at an equal distance from the record. The printer according to the invention satisfies these requirements in establishing the locations of the exposure and the charging stations.
A sheet feeder for supplying a sheet of record paper toward the record will now be described. In one sheet feeder known in the art, a stack of record sheets is disposed on a movable bottom plate of a tray, and raising member which is urged by a spring bears against the bottom plate to push it up so that an uppermost one of the record sheets in the stack is urged against a feed roller, which then feeds such sheet. In another arrangement in which the tray projects out of the arrangement, the tray is provided with a top cover which can be selectively opened and closed in order to prevent a marring of the record sheets.
In the paper feeder of the type described above, in order to allow the record sheets to be replenished, the raising member must be depressed initially by means of an operating lever to open the top cover, whereupon a supply of record sheets is fed into the tray, followed by closing the cover and releasing the raising member to return it to its upper location. In an alternative arrangement employing a cassette in which a tray is detachably mounted on the printer, it is also necessary to withdraw the cassette from or insert it into the printer, in addition to the operation mentioned above. Thus, a replenishment of record sheets is a troublesome operation.
In the paper feeder described above, it is necessary to provide a sensor which detects the presence or absence of a record sheet or sheets on the bottom plate, and another sensor which detects whether the bottom plate is urged toward the feed roller to enable a normal sheet feed operation whenever the top cover is closed.
Also, a sheet feeder of a type is known in which the bottom plate moves upward to urge the record sheet against the feed roller, which then rotates to partly feed a plurality of record sheets, but delivers only one of the record sheets out of the tray by utilizing a friction pad having a different coefficient of friction. In a sheet feeder of this type, the possibility must be taken into consideration that an additional supply of record sheets may not be properly oriented within the tray. The leading ends of a plurality of record sheets are held between the feed roller and the friction pad. If the bottom plate is allowed to move down under this condition, these record sheets remain in position where their leading ends are held between the roller and the pad without moving down in following relationship with the movement of the bottom plate. If another supply of record sheets is subsequently placed on top thereof, there results an inconvenience that the record sheets which have their leading ends held between the roller and the pad as well as an uppermost one of the sheets are also fed. It is to be mentioned here that located above the sheet feeder is the record/developing unit, above which is disposed the cleaning unit. The purpose of the cleaning unit is to remove and recover any developer which remains on the surface of the record after the transfer step. Accordingly, when the unit is either entirely or partly withdrawn for purpose of replacing the record or replenishing the developer, oscillations or impacts may cause the developer which is recovered by the . cleaning unit to fall down over the sheet feeder, in particular over the roller pair, the feed roller and the friction pad. If such developer is deposited thereon, the coefficients of friction of the surfaces of these rollers and pad may change, preventing a normal feed operation. In addition, developer may deposit on the record sheet to produce a marred copy. In addition, during a movement of the record, a powder image formed on the surface thereof may produce a floating toner which mars the record sheet or rollers. It is to be noted that the record sheet, when it is delivered out of the tray, is brought into close contact with the record, and it is necessary that the record sheet be fed in proper orientation.
The printer of the invention employs a fixing unit of roller type. A fixing unit of roller type generally includes a separation claw which is provided for purpose of preventing a record sheet from being wrapped around a fixing roller after the image has been fixed. In the conventional arrangement, the leading end of the claw is maintained in contact with the fixing roller. When the claw is maintained in contact with the roller, the leading end of the claw which is held in contact with the roller serves to scrape the toner, as it is deposited on the roller, thus gradually forming a toner deposition thereon. Accordingly, after a prolonged period of use, the toner deposition causes the leading end of the claw to be removed from the fixing roller, with result that a normal separation of the sheet by the claw is prevented, thereby causing the record sheet to be engaged with the claw, or producing a jamming. In addition, because the leading end of the separation claw is maintained in contact with the roller, a Teflon coating of the roller surface may be damaged to prevent a satisfactory fixing operation in such region, thus causing white streaking in the fixed image to cause a degradation in the image quality.
It is then proposed to provide an electromagnetic drive such as plunger which is energized by an electrical signal when it is desired to separate a record sheet, thereby causing an . angular movement of the claw into contact with the roller while normally maintaining it removed from the latter. However, the provision of a separate drive, with an associated control, to cause a movement of the claw toward or away from the roller results in a mechanically and electrically complex arrangement and an increased space requirement to cause an increased cost.
After it is charged, the record should be neutralized for subsequent use thereof. It is also necessary that the transfer paper should be securely separated from the record after a visual image has been transferred to the paper.
To assure a satisfactory cleaning operation with the cleaning unit mentioned above, it is necessary that a constant spacing be maintained between the sleeve and the record surface. In other words, the contact between the fibers implanted on the sleeve surface and the record surface must be maintained uniform, since otherwise a non-uniformity occurs in the cleaning effect. Any residual toner which remains as a result of a failure of cleaning operation or insufficient cleaning operation prevents a satisfactory performance of a next record operation. To overcome such difficulty, it has been the prior art practice to provide bearings on the opposite ends of the sleeve and having a diameter which is slightly greater than the diameter of the sleeve so that these bearings are disposed in abutment against the record surface during their rotation to maintain a constant spacing between the sleeve and the record surface. However, this requires a complex mechanism and causes an increase in the cost. In addition, a disadvantage results that foreign matters such as toner may be deposited in the area of contact between the bearings and the record to cause a gradual decrease in the accuracy of the spacing maintained. According to the invention, a recording is made on the record in the form of an endless belt. In this technique, it is necessary that a junction or joint in the record be treated as a non-record area. Hence, during a record operation, there must be provided some means which detects such junction or joint on the record or the location thereof where the recording operation may be initiated, thus preventing a junction or joint region from being used in its recording operation. It is also to be recognized that it is undesirable that it takes an increased length of time to move the record before the recording operation can be initiated or that the record continues to move after the intended recording operation has been completed.
As a safeguard arrangement associated with the apparatus to form an image, the prior art employs a paper end sensor which detects the presence or absence of record sheets, a register sensor which detects a feed mistake and a jamming sensor which detects the occurrence of a jamming in order to take a suitable measure. By way of example, if the paper end sensor detects the absence of any record sheet, feeding of a record sheet is prevented while simultaneously preventing the register sensor and the jamming sensor from operating, in order to prevent the apparatus from assuming an abnormal condition. In the event the register sensor detects a feed mistake, the jamming sensor is prevented from operating and from acting to provide a safeguard action, again in order to prevent the apparatus from assuming an abnormal condition. However, in a printer using a record in the form of an endless belt, the sequence control is based on a sync signal. If a paper end (exhaustion) or a feed mistake occurs, the subsequent detecting operations are prevented in the prior art arrangement. Accordingly, separate sequential operations must be performed as the situation may be, disadvantageously resulting in a complex control.
Specifically, when the motor starts, a single sync signal is produced per revolution of the record, thus achieving a synchronization between the record and the sequence. At a given time after the initial sync signal is supplied, a write enable signal is produced, which permits the entry of an image to be initiated. The feed roller is driven to supply a record sheet. As the record sheet reaches the location of the register sensor, the drive to the feed roller and the conveyor roller is interrupted, maintaining the record sheet in standby mode at a given location. Subsequently, in response to a signal from a timer which indicates that a given time interval has passed since the occurrence of the write enable signal, the conveyor rollers are restarted to register the leading end of the record sheet with the leading end of the image. The block A.sub.1 which effects only the detection of a feed mistake, the block A.sub.2 which effects the detection of both a feed mistake and a jamming, the block B which effects the detection of a jamming alone and the block C in which no detection is performed must be separately controlled. In this manner, there results a complex control even though the control would be facilitated if common terms are used in the individual blocks. In the prior art arrangement, if a paper end (exhaustion) is detected in the block A.sub.2 and a feed operation is prevented, the detection of a feed mistake must be prevented. Alternatively, if a feed mistake is detected, the detection of a jamming and an associated safeguard operation must be prevented in the block B which is used to deliver the preceeding sheet externally of the printer.
The temperature of the heater used in the fixing unit is detected by means of a thermistor, and the power supply is controlled to maintain a constant temperature. In the prior art arrangement, a thermal fuse is connected in a loop which is used to energize the heater since there is a likelihood for the heater to be excessively heated in the event the thermistor is broken or an abnormality occurs in the temperature detection circuit. If the thermistor is broken, a potential increase or decrease due to the breakage of the thermistor is detected. In this manner, any abnormality which results from the breakage of the thermistor is immediately and automatically detected. However, there is no remedy when the thermal fuse is blown. A printer is available which initiates a time counting operation from the time when the power source for the heater is turned on, and determines automatically the presence of an abnormality in the power system, including a blowout of a thermal fuse unless the detected thermistor temperature reaches or exceeds a given value within a defined time limit. However, this involves a loss time since the determination is rendered after a warm-up period for the heater temperature, which is preset for a normal operation, since the power source for the heater is turned on subsequent to the turn-on of the power supply to the apparatus The determination of the occurrence of any abnormality is normally executed only immediately after the power supply to the apparatus is turned on, and there is no remedy after the warm-up period for the temperature has passed if the thermal fuse is blown out. To accommodate for this possibility, it is necessary that the operational sequence of the printer includes an abnormality detecting routine which monitors the time periods during which the heater is energized and deenergized and which determines the occurrence of an abnormality in the power system associated with the heater if the detected thermistor temperature is below a given value when it should exceed the latter value. In this manner, the detection of the abnormality is complex, adding to the tasks which must be processed in the printer sequence.
In the printer, the power supply for the control unit as well as the power supply for other mechanical components are turned on when the so-called power switch is turned on by an operator operation or by the closure of a power relay switch in response to an external turn-on signal applied, whereby various devices are preset in a standby mode. Accordingly, the various devices assume given conditions at the start of a printer operation, assuring a smooth initiation of a print operation. When the print operation is completed, these devices return to their standby conditions. Accordingly, it is preferable that the power supplies be turned off during the standby mode by disconnecting the power switch by an operation of an operator or by the opening of the power relay switch in response to the removal of an external turn-on signal applied. However, if the turn-off of the power supplies occurs during the print operation, the presence of a record sheet which fails to be delivered or an incomplete cleaning operation may cause a malfunctioning or a degraded copy quality when the power supplies are turned on the next time. In particular, considering the record, it will be noted that the ozone produced by the charger may remain within the charger casing to degrade the surface of the record in the region where an image to be formed, causing a degradation in the quality of a latent image formed during a subsequent operation.
The record is formed with the sub-scan sync mark, while the drive system for the record is provided with the sync mark detector so that a timing pulse which is based on the detection of such mark and thus is synchronized with the movement of the record may be counted to control the timing of sheet feed, charging, exposure and transfer operations in accordance with preset counts. However, the record may slip with respect to the drive roller, and accordingly the count is initiated to a given value, usually cleared to zero, each time the mark is detected. However, if the magnitude of slip is high, a deviation in the timing may be caused, which cannot be prevented by the initialization alone. To deal with this problem, the prior art provides a timer which is triggered at the time the mark is detected, and unless the mark is detected immediately after the time has timed out, a determination is rendered that there has occurred a timing error, thus indicating the occurrence of an abnormality, by energizing a display or the like to indicate the necessity for an inspection. However, in the prior art practice, of detecting such error, a hardware time such as time limit circuit or a timer integrated circuit is connected to the central unit which performs the timing control, resulting in an increased cost. In addition, the components C and R used to determine the time limit may involve a certain tolerance, which must be adjusted. Also, the temperature causes a variation in the time limit, which therefore must be chosen to be greater than is necessary, resulting in a degraded accuracy in the detection of abnormal slip occurred.
It is also to be noted that the slip between the record and the drive roller may occur at a variable point in time, whereby the phase difference between the detection of the mark and the occurrence of the timing pulses is not uniform. The timing pulse may appear between successive detections of the mark or may appear immediately before or after the detection of the mark. A uniform initialization results in an initial offset between the initial count and the position of the record which varies from instance to instance depending on the time when the detection of a mark occurs and the period of the timing pulse, causing a shift in the control timing which gives rise to a displacement of the location of an image on the record.
A fixing unit which achieves a fixing of a toner image by the application of heat and pressure to a record sheet carrying an unfixed toner image and passing between a pair of rollers requires the provision of a cleaning unit associated with the fixing roller. The usual practice has been to employ a cleaning pad which is brought into abutment against the peripheral surface of the fixing roller. In this instance, the pad abuts against the fixing roller with a uniform force as viewed in the direction of rotation of the roller. Accordingly, the toner deposited on the fixing roller tends to be gradually accumulated on the advanced side of the pad, with the accumulated toner falling down to mar the transfer sheet or a pressure roller disadvantageously.
In the prior art practice, there is a proposal to provide an apparatus for inverting record sheets, which apparatus is located adjacent to a delivery port of the printer in order to permit a paging of record sheets as they are delivered from the printer. However, in the prior art practice, a fitting is employed to construct an inverting apparatus which is perfectly fixed in position. This presents a great inconvenience when a paper jamming occurs at the delivery port or in the fixing unit adjacent thereto or in the event a repair of adjacent parts is required.
When the record is in the form of an endless belt having a joint therein, it is necessary to treat the joint as a nonrecord area. Accordingly, it is necessary to provide some means which detects a starting position for the record or the joint therein so that an image formed can be on the record while avoiding the joint. In contradistinction to the remainder, an area involving the joint is uneven, has a reduced mechanical strength, or may have a photoconductive layer which is liable to exfoliation from a base layer. Hence it is undesirable that the record be stopped at an arbitrarily chosen position. It is also undesirable to spend a length of time before the formation of an image on the record is initiated or to allow the record to continue its movement after the desired image has been formed thereon. If the record is maintained stationary for a prolonged length of time during the formation of an image and if a certain area of the record is located adjacent to a step or station which has an adverse influence upon the formation of an image, such area may be influenced in some way by such step to prevent an image from being formed therein or to result in an image of greatly degraded quality during the next cycle of operation.
A printer employing a record in the form of an endless belt requires the provision of a safety unit which assures a synchronization between a conveying operation of the record and the progress of a printing operation as well as the detection of the presence or absence of a record sheet. A conventional safety unit for printer is known including first detection means which detects the exhaustion of a transfer sheet, second detection means which detects the occurrence of a feed mistake, and third detection means which detects the occurrence of a jamming. Signals from these detection means are utilized to provide an appropriate remedy. For example, when the exhaustion of a transfer sheet is detected by the first detection means, a feed operation for the transfer sheet is prevented while simultaneously disabling the operation of the second and the third detection means, thus avoiding that the priner assumes an abnormal condition. In the event the second detection means has detected a feed mistake, the operation of the third detection means as well as resulting remedy are disabled, thus preventing the printer from entering an abnormal condition. However, in the printer employing an endless belt as the record, it is necessary to perform a sequence control on the basis of a sync signal. In the event the exhaustion of a transfer sheet or a feed mistake occurs, the subsequent operation is disabled in the prior art, so that separate individual sequential operations are required subsequently, resulting in a complex control. By way of example, a path of movement of a record sheet or a transfer sheet may be divided into a plurality of blocks including a block A where only the detection of a feed mistake takes place, a block B where the detection of both the feed mistake and the jamming takes place, a block C where only the jamming is detected and a block D where no detection is made. It will be seen that separate controls are required for each of these blocks. Even though the control will be greatly facilitated if items are provided which are common to all of these blocks, the actual control results in a complex arrangement. Specifically, in the convention arrangement, if the exhaustion of a record sheet is detected in the block B and a feed operation for the record sheet is not performed, the detection of the feed mistake must be disabled. If the feed mistake is detected, the detection of a jamming and the resulting remedy must be disabled in the block C where the preceding sheet has to be delivered out of the printer.
It will be understood that the temperature of the fixing roller is detected by means of a thermistor, and power control is effected to produce a constant temperature. In the event an abnormal condition occurs in a temperature detector circuit including the thermistor or in the event of occurrence of a breakage of the thermistor, the likelihood of an associated heater being overheated must be prevented by connecting a thermal fuse in a loop which is used to energize the heater. The occurrence of a breakage of the thermistor is detected by an increase or decrease in the potential which results from such breakage. In this manner, the breakage of the thermistor can be immediately and automatically detected. However, there is no immediate reaction to the blowout of the thermal fuse. A printer is provided in the prior art which automatically determines the existence of an abnormality in the power system, including the blowout of the thermal fuse, unless the thermistor temperature exceeds a given value within a fixed time interval as the time is counted from the turn-on of the heater. However, this arrangement involves a time lag since the determination is made after a rising time which is normally required for the heater temperature to reach a given value after the power supply therefor has been turned on. In addition, such abnormality determining flow chart is executed only immediately after the power supply for the heater is turned on, and no remedy is provided for the blowout of the thermal fuse after the temperature has reached its normal value. To accommodate for such possibility, the control sequence of the printer must include an abnormality detecting flow chart which monitors the time intervals during which the heater is energized and deenergized and which decides the existence of an abnormality in the heater power system if the thermistor temperature is less than a given value when it should exceed the latter. This complicates the detection of abnormalities in the heater power system, and adds to the tasks in the control sequence of the printer.
In a printer, a power switch is turned on by an operator, or an ON signal is externally applied to close a relay switch associated with a power source, whereby the power supply for the control unit and other devices are turned on, establishing the various parts of the printer in a standby mode. In this manner, the various devices assume given conditions at the start of a printing operation, allowing a smooth initiation of the operation. The completion of the printing operation returns the printer to a similar standby mode, so that it is desirable that during a standby mode, the power switch be turned off by an operator or the ON signal which is externally applied is removed to open the relay switch associated with the power source, thereby turning the power supply off. However, if such a turn-off occurs in the course of the printing operation, the presence of a record sheet which remains undelivered or the incomplete cleaning operation may cause a malfunctioning of the arrangement when the power supply is turned on for the next time, causing a degradation in the quality of the copy. In particular, ozone produced by the charger may remain within the casing of the charger, thereby degrading the quality of the surface of the record in a region where an image is to be formed, and thus degrading the quality of a latent image to be formed subsequently.
Where a record having a joint therein is used, an area of the record which includes the joint cannot be utilized as a region where an image is to be formed. Consequently, during the printing operation, it is necessary to control the operation of the printer so that image information be always supplied to an image region of the record which is free from the joint. At this end, the record is formed with a mark intended to produce a sync signal. The mark can be detected to derive a sync signal which is in turn utilized to control the operation of the printer.
However, a detector which is provided to detected the mark on the record also happens to produce an output signal in response to a flaw or dust on the record. Consequently, if an output from the detector is utilized as a sync signal which is used to control the operation of the printer, a proper operation of the printer may be prevented since output signals which are produced in succession by the detector in response to a number of flaws and dusts may prevent a sequential transfer between successive steps even though the time passes, thus resulting in a failure to complete the printing operation.
In the printer, the record in the form of an endless belt is formed with a sync mark, while a belt drive system is coupled with an encoder. Pulses from the encoder or timing pulses synchronized with the movement of the belt are counted, choosing the detection of the mark as the start point. The resulting counts are utilized to determine the timing when the sheet feeding, charging, exposure and transfer steps are to be performed. However, the belt may slip relative to the drive roller. Accordingly, the count is reset to a given value, or usually cleared to zero each time the mark is detected. However, a phase difference between the detection of the mark and the occurrence of a timing pulse may result from a slip of the belt with respect to the drive roller, and its magnitude is not uniform. In certain instance, a timing pulse may appear between successive detections of the mark while at other times, the timing pulse may appear immediately before or after the detection of the mark. Hence, if the count is reset to a constant value, an initial displacement of the timing pulse relative to the detection of the mark or a relative displacement between the initial count and the position of the record may differ from initialization to initialization, causing a displaced timing in the printing operation, which results in the displacement of a picture frame.
If there occurs a large magnitude of slip, it cannot be compensated for by the initialization. To accommodate for this, there is provided an arrangement in the prior art in which a timer is triggered at the time when a mark is detected, and unless the mark is detected immediately after the timer has timed out, it is decided that there has occurred a timing error, causing a display to indicate an abnormality and requiring an inspection. However, to detect an error in this manner, a central control unit which controls the timing of various operations in the printer must be connected with a hardware time such as a time limit circuit or IC timer, resulting in an increased cost. In addition, an adjusting circuit is required which compensates for tolerances in the value of C and R components which are used to define a time limit, requiring an additional adjustment. In addition, the time limit may shift with temperature. This requires that the time limit be chosen to be longer than necessary, resulting in a coarse detection of the slip or abnormality.